Triiodothyronine Acutely Stimulates Glucose Transport into L6 Muscle Cells Without Increasing Surface GLUT4, GLUT1, or GLUT3

Abstract

Background: Thyroid hormones (THs) act genomically to stimulate glucose transport by elevating glucose transporter (Slc2a) expression and glucose utilization by cells. However, nongenomic effects of THs are now emerging. Here, we assess how triiodothyronine (T 3 ) acutely affects glucose transport and the content of GLUT4, GLUT1, and GLUT3 at the surface of muscle cells, and possible interactions between T 3 and insulin action. Methods: Differentiated L6 myotubes transfected with myc-tagged Slc2a4 (L6-GLUT4myc) or Slc2a1 (L6-GLUT1myc) and wild-type L6 myotubes were studied in the following conditions: control, hypothyroid (Tx), Tx plus T 3 , Tx plus insulin, and Tx plus insulin and T 3 . Results: Glucose uptake and GLUT4 content at the cell surface decreased in the Tx group relative to controls. T 3 treatment for 30 minutes increased glucose transport into L6-GLUT4myc cells without altering surface GLUT4 content, which increased only thereafter. The total amount of GLUT4 protein remained unchanged among the groups studied. The surface GLUT1 content of L6-GLUT1myc cells also remained unaltered after T 3 treatment; however, in these cells glucose transport was not stimulated by T 3 . In wild-type L6 cells, although T 3 treatment increased the total amount of GLUT3, it did not change the surface GLUT3 content. Moreover, within 30 minutes, T 3 stimulation of glucose uptake was additive to that of insulin in L6-GLUT4myc cells. As expected, insulin elevated surface GLUT4 content and glucose uptake. However, interestingly, surface GLUT4 content remained unchanged or even dropped with T 3 plus insulin. Conclusions: These data reveal that T 3 rapidly increases glucose uptake in L6-GLUT4myc cells, which, at least for 30 minutes, did not depend on an increment in GLUT4 at the cell surface yet potentiates insulin action. We propose that this rapid T 3 effect involves activation of GLUT4 transporters at the cell surface, but cannot discount the involvement of an unknown GLUT.

Thyroid hormone inhibition in L6 myoblasts of IGF-I-mediated glucose uptake and proliferation: new roles for integrin αvβ3

“…Very recently, shortterm triiodothyronine (10 Ϫ9 M) exposure was shown to normalize glucose transport in thyroid hormone-deprived L6 myotubes; the effect was additive to that of insulin. In these studies, there was no translocation of insulin-sensitive glucose transporters to the plasma membrane (69).…”

“…Glucose uptake in skeletal muscle cells has been well characterized in the last 20 years by several groups (4,5,24,59,61,69). We carried out experiments to confirm our experimental model, measuring glucose uptake in L6 myoblasts induced by 0.1 nM insulin or 10 nM IGF-I ( Fig.…”

Calcium signaling recruits substrate transporters GLUT4 and CD36 to the sarcolemma without increasing cardiac substrate uptake

“…This method is also suitable for detecting cell surface GLUT4, notwithstanding that GLUT4 expresses only a single extracellular lysine (at position 50). Accordingly, we (22) and others (11,42) have shown that insulin stimulates the binding of sulfo-NHS-LC-biotin to GLUT4. In the present study, oligomycin, a well-established inducer of GLUT4 and CD36 translocation (32), was used as positive control to validate the suitability of the cell surface biotinylation protocol and also of the other methods to detect surface GLUT4 and CD36.…”

Hypothyroidism Is a Risk Factor for New-Onset Diabetes: A Cohort Study

“…The association among hypothyroidism, subclinical hypothyroidism, and DM is described in the literature but remains to be better established. Insulin resistance is found in patients with hypothyroidism (18) and decreased glucose uptake into muscle and adipose tissues exhibited in hypothyroid state in vitro (19). Impaired translocation of GLUT4 glucose transporters on the plasma membrane in patients with hypothyroidism and subclinical hypothyroidism (20) as well as downregulation of the hepatic glucose transporter GLUT2 (21) have been demonstrated.…”